1. Technical Field
The present invention relates to apparatus for injecting fuel into internal combustion engines, particularly compression ignition engines.
2. Background
The common means of injecting fuel into modern diesel engines can be divided in two functionally different groups: mechanically actuated systems and common rail systems. The majority of heavy-duty diesel engines for commercial vehicles utilize mechanically actuated, electronically controlled unit injector/unit pump systems. The light duty diesel engine market is dominated by either pump-line-nozzle mechanically actuated fuel injection systems (FIE) or so called high pressure common rail systems.
There are several types of mechanically actuated unit injectors/pumps. All of them are capable of creating very high injection pressures with relatively good hydraulic/mechanical efficiency, which is one of their most important advantages over the common rail systems. Another advantage is significantly better durability. Durability of high pressure common rail systems is inferior to mechanically actuated systems largely due to constant exposure of its elements to maximum fuel pressure which is required for injection. Yet another significant advantage of mechanically actuated unit injection systems is their natural ability to achieve favorable injection rate development during a single injection. High pressure common rail systems cannot easily provide such injection characteristic and, when their inherent square-shaped injection trace pattern becomes desirable in some engine operating points, the contemporary unit injectors with a direct nozzle control valve can shape an injection in this way just as well. This affords the latter systems better flexibility in injection rate shaping.
On the other hand, high pressure common rail systems have certain advantages over the mechanically actuated injection systems. Among those most important for the commercial vehicle engines have almost unlimited injection timing flexibility and ease of achieving multiple injections. Such an ability of a fuel injection system gains importance with the introduction of various types of diesel exhaust aftertreatment devices and advances in the development of alternative combustion processes like HCCl. The reliance of the mechanically actuated systems on a cam driving the pumping plunger can significantly restrict their ability to fulfill the requirements to injection timing and fuelling of multiple injections. The other advantage of a high pressure common rail system over a mechanical unit injection system can be lower parasitic drive power losses when operating at very low engine loads and idle. At such conditions, high pressure common rail systems also have better accuracy of fuel delivery than a mechanically actuated unit injection system with a large plunger diameter. Finally, mechanically actuated unit injection systems can be a source of excessive mechanical noise generated by both the FIE itself and the drivetrain transmitting torque to actuate the system. Such excessive noise is especially conspicuous at engine idle. The operation of the high pressure common rail systems does not significantly contribute to the total engine noise at any operating point.
U.S. Pat. No. 6,247,450 by Jiang discloses a system consisting of a mechanically actuated unit injector with a control valve and a common rail. In that system, the common rail pressure is regulated at relatively low levels and the fuel under this pressure can be fed into the unit injector through a metering orifice that is opened at a certain retracted position of the plunger of the unit injector, and closed at other plunger positions. Variation of common rail pressure and the duration of opening of the metering orifice determine the amount of fuel filling the plunger chamber. During a pumping stroke of the plunger, the metering orifice is closed and fuel is pressurized in the plunger chamber, which is appropriately sized to allow for necessary injection pressure to be reached. The plunger chamber is connected to the inlet of a conventional spring-closed nozzle via a control valve. Upon reaching a required pressure level, the control valve can be opened to transmit the pressurized fuel to the nozzle and commence injection. To end injection, the valve closes and the nozzle is closed by the return spring.
Such a system relies on the plunger being stationary at the maximum lift and keeping the pressure created during the pumping stroke to provide flexibility in injection timing. Fuel injection cannot possibly take place during most of the retraction and pumping strokes of the plunger due to the metering orifice being closed. Clearly, the system is not designed to inject at any other time but when the plunger is close to the maximum lift, because even if the control valve were opened during the fuel metering phase and common rail pressure were set above the spring opening pressure of the nozzle, the pressure drop across the metering orifice that is necessary to achieve the fuel metering function of the system, would have prevented injection.
Apart from a restricted injection timing range, the system of the U.S. Pat. No. 6,247,450 suffers from a number of other drawbacks, namely, unfavorable shape of injection rate trace both in the beginning and end of injection, restricted range of injection pressures etc.
The other prior art FIE which can be considered relevant to the present invention is that referred to as pressure/time metering unit injection system introduced into the market by Cummins Inc. Examples of such system can be found in U.S. Pat. Nos. 3,544,008, 4,092,964 and 5,445,323. A system of this type contains a pressurized fuel common rail feeding unit injectors. However, the function of the common rail is not to directly inject fuel into the engine, but to facilitate fuel metering into the plunger chamber which will be displaced through the nozzle during the pumping stroke of the plunger. Such systems thus have a limited injection timing range and need to utilize the mechanical actuation every time an injection is due.